Typically, the exhaust gases of metal smelteries are hot gases of 1100.degree.-1400.degree. C., and they contain solid material, i.e. dust which is partly in a molten state, and gas components which during cooling, e.g. down to 200.degree.-400.degree. C., condense to a solid phase.
Usually, the treatment of exhaust gases from this kind of processes has been arranged by cooling the gas first in a waste heat recovery boiler generating saturated or sometimes superheated steam and by separating, subsequent to the waste heat boiler, solids from the gas for example in an electric filter. In smelteries, the use of a steam boiler is based on the possibility of generating electricity by means of a steam turbine to satisfy the demand of the plant and also to be sold.
Most metal sulfide smelting processes employ a smelting furnace structure in which the discharge of the exhaust gases is easiest and simplest effected upwards through an opening provided in the roof of the furnace. U.S. Pat. No. 4,087,274 discloses a smelting furnace from which the exhaust gases are removed via an opening in the roof of the furnace.
This arrangement, however, involves a risk if the steam boiler or its first heat surfaces are constructed directly above the smelting furnace extending upwards from the opening provided in the roof of the furnace. Bursting of a steam boiler tube causes a water leakage which results in a risk of explosion in the smelting furnace if the water spraying out from the leakage point runs down to the smelt.
To solve the above problem, the boiler located on top of the furnace could be provided with a superheater. The medium flowing in these heat surfaces is steam and the section located above the furnace serves as a superheater for steam. The more risky heat surfaces, i.e. the evaporators containing boiler water, would be installed further off and not directly above the smelt. In practice, a construction of this kind is, however, impossible, for example because one of the biggest problems in cooling of the gases is the sticking of dust to the heat surfaces which results in a tendency of the surfaces to clogg which in turn increases the heat transfer resistance. An increase in the temperature of the surface intensifies this phenomenon and therefore the heat surfaces of this kind of boilers are usually designed to give an as high cooling effect as possible and to serve as evaporating surfaces generating saturated steam instead of hot superheater surfaces. If necessary in some applications, the steam produced in this kind of boilers is superheated in a separate superheating boiler prior to the steam turbine. Another drawback of this application is the fact that at the steam pressures concerned (i.e. less than 100 bar) the thermal energy for superheating compared with the thermal energy for evaporation is so low that superheating alone would not suffice for achieving adequate cooling in the boiler portion disposed above the furnace. The use of a steam pressure exceeding 100 bar would, on the other hand, result in the temperature of the evaporation surfaces rising too high for example in view of cleaning.
A conventional boiler arrangement used in smelteries is a horizontal boiler arranged at a side of the smelting furnace, thereby avoiding the risk of an explosion caused by a water leak. A similar boiler arrangement is used, e.g. in a smelting process disclosed in U.S. Pat. No. 4,073,645. The arrangement has proved to operate well but the boiler structure is expensive and space consuming and thus, on the whole, the use of this kind of technique impairs the economy of the heat recovery from the exhaust gas.